Molecular characterization of prostate cancer has revealed distinct subtypes, but the biologic implications and translational impact of these are still unclear. We recently defined a distinct molecular subclass of prostate cancer characterized by the combination of mutations in the ubiquitin ligase SPOP, and deletion of the nucleosome remodeler CHD1. However, the mechanisms by which SPOP mutation and CHD1 deletion collaborate to drive prostate tumorigenesis remain unknown. The overall objective of this proposal is to define the mechanisms by which SPOP mutation and CHD1 deletion collaborate to drive prostate tumorigenesis. Using novel models, our preliminary data demonstrate that SPOP mutation alters expression of proteins that are functionally and physically associated with the androgen receptor (AR), a critical driver for prostate cancer and key therapeutic target, and alters AR-driven gene expression. Furthermore, we have found that CHD1 deletion fundamentally reprograms the AR-cistrome and transcriptional program in prostate cells, diverting away from a growth suppressive AR program and towards an oncogenic program. These results suggest that mutation of SPOP stabilizes key substrates involved in AR activity, and that CHD1 deletion specifically collaborates with these changes in the AR- transcriptional complex to promote oncogenic transcription and prostate tumorigenesis. This project will elucidate the molecular details underlying these phenomena through the following Aims: 1) establishing the collaborative impact of SPOP mutation and CHD1 deletion on the pathogenesis of prostate cancer, 2) defining the alterations to the AR-transcriptional complex mediated by SPOP mutation, and 3) determining the alterations induced by SPOP mutation that are critical for collaboration with CHD1 loss and reprograming of the AR transcriptome. To accomplish this, we will leverage unique, biologically and clinically relevant model systems, innovative approaches to proteomic and transcriptomic discovery, and data from human prostate cancer samples. This project will define the critical transcriptional processes in SPOP mutant / CHD1 deleted prostate cancer and the broader applicability across prostate cancer subtypes, and provide the foundation for precision clinical trials targeting this subclass.